1. Field of the Invention
The present invention relates to a battery-charging device for a stand-alone generator system having a MPPT (maximum power point tracking) function and a method thereof. Particularly, the present invention relates to the battery-charging device and the method thereof applied to electrical generation equipment of wind energy, solar energy or other renewable energy, with the battery-charging device including a DC/DC converter and a control circuit. More particularly, the present invention relates to the control circuit of the battery-charging device provided with four operation modes, with one of the operation modes selected and actuated by the output of wind energy or solar energy power and the state-of-health of batteries so as to increase the utilization of wind energy and solar energy.
2. Description of the Related Art
A conventional small stand-alone wind energy or solar energy generator system directly supplies to loads, and any redundant electric power will charge batteries for storing energy. The batteries will supply electric power to the load when the electric power supplied from the generator system cannot satisfy the power demanded by the load. Generally, such a stand-alone generator system is only designed with small-capacity equipment and is suitable for remote areas where no utility power can arrive. The stand-alone generator system is frequently used in the environment with a shortage of an electric power resource. Hence, there is a need for increasing a ratio of energy utilization of such a stand-alone generator system.
In some circumstances, the electric power supplied from the stand-alone generator system of wind energy or solar energy is lower than that required by loads. Accordingly, the stand-alone generator system of wind energy or solar energy is equipped with batteries and a battery-charging device. In order to effectively utilize the electric power stored in batteries of the stand-alone generator system, the power generated from the stand-alone generator system must rapidly charge the batteries which can perform as an electric power source of loads. Disadvantageously, rapidly improper charging may cause damage to batteries and may shorten the life of the battery.
There are several major battery-charging methods including: a constant current charging method, a constant voltage charging method, a hybrid constant current/constant voltage charging method and a pulse current charging method.
Each of the above-mentioned charging methods has several advantages and drawbacks. For instance, the constant current charging method has a drawback of overcharge occurrence when the battery voltage is high. In addition, the constant voltage charging method has a drawback of causing a large charging current, when the battery voltage is low, thus damaging the battery. The hybrid constant current/constant voltage charging method can improve the drawback of a large charging current when the battery voltage is low. However, such a charging method requires a longer charging time. As to the pulse current charging method, damages to the battery may be relatively small.
In general, a conventional battery-charging method of the stand-alone generator system (for example: a wind energy or solar energy power generator system) having a MPPT function utilizes the two-stage constant current/constant voltage charging method, namely the hybrid constant current/constant voltage charging method. When the power generated by the electric power generator system is lower than the summation of the load power and the maximum charging power (Pb) of the constant current charging method for a battery, the generator system is operated in a MPPT mode. Conversely, when the power generated by the electric power generator system is higher than the summation of the load power and the maximum charging power (Pb) of the constant current charging method for a battery, the power generated by the electric power generator system is limited to be the summation of the load power and the maximum charging power (Pb) of the constant current charging method for a battery to avoid the high charging current damaging the battery. In this circumstance, the MPPT function of the generator system is terminated, so that the electric power generator system cannot supply the maximum electric power and the redundant electric power cannot be effectively utilized.
Referring initially to FIG. 1, a curve diagram of an output electric power P(W) of a conventional stand-alone generator system having a MPPT function is shown. When the stand-alone generator system is a solar energy power generator system, curves P1, P2, P3, P4, as shown in FIG. 1, represent the output power of the solar energy generator system in relation to its output voltages under various solar illuminations. The curve P4 represents the output power of the solar energy generator system under a maximum solar illumination, while the curve P1 represents the output power of the solar energy generator system under a minimum solar illumination. When the stand-alone generator system is a wind energy generator system, curves P1, P2, P3, P4 represent the output power of the wind energy generator system in relation to its rotational speeds under various wind speeds. The curve P4 represents the output power of the wind energy generator system under a maximum wind speed, while the curve P1 represents the output power of the wind energy generator system under a minimum wind speed. With continued reference to FIG. 1, when maximum power sections of the curves P3 and P4 are greater than the summation of the load power and the maximum charging power (Pb) of the constant current charging method for a battery, the power generated by the electric power generator system is limited to be the summation of the load power and the maximum charging power (Pb) of the constant current charging method for a battery. Disadvantageously, the output electric power generated by the electrical power source, as best shown in a meshed area in FIG. 1, cannot be effectively utilized while located in a maximum power area (i.e. the redundant electric power area) of the curves P3 and P4.
Another conventional solar energy pulse battery-charging device, as described in Taiwanese Patent Publication No. 566745, has a battery-charging method similar to the above-mentioned method. Another conventional charging method of a battery-charging device having a MPPT function, as described in U.S. Patent Publication No. 2004/0174939, discloses a constant voltage generated from solar power energy to supply a battery and loads via a battery-charging device. The battery-charging device can be controlled to execute the MPPT function if the output power of the battery-charging device is less than the summation of the load power and the battery-charging power. Conversely, if the output power of the solar energy generator system is greater than the summation of the load power and the battery-charging power, the battery-charging device is controlled to terminate the MPPT function and to set the output power of the solar energy generator system equal to the summation of load power and battery-charging power so as to maintain outputting a constant voltage. In this manner, such a battery-charging method is a constant voltage charging method.
The battery-charging device of U.S. Patent Pub. No. 2004/0174939 is successful in outputting a stable constant voltage, avoiding the damage of severe voltage variation to loads and simplifying the entire structure. However, the difference between the battery voltage and the output voltage of the battery-charging device is large when the battery is situated in low stored energy, and it may cause an overcharging current if the battery-charging device is operated in a constant voltage charging mode. Disadvantageously, the overcharging current may not only shorten the life of battery but also may damage the battery-charging device. Briefly, there is a need for improving the battery-charging method applied in U.S. Patent Pub. No. 2004/0174939 so as to avoid damaging the battery-charging device and the battery. In addition, there is a need for improving problematic aspects while incorporating the MPPT function with other battery-charging methods.
Taiwanese patent publication No. 200825278 discloses a battery-charging method of a wind energy generator system having a MPPT function. However, such a battery-charging method requires utilizing a DC/DC converter and a pulse control (pulse generating) circuit and includes five operation modes of the battery-charging device. Disadvantageously, the circuit hardware and the control method applied therein are complicated.
As is described in greater detail below, the present invention provides a battery-charging device for a stand-alone generator system having a MPPT function. The battery-charging device includes a DC/DC converter and a control circuit. The control circuit is used to control the DC/DC converter performing four operation modes. When a generated output power of the electrical power source of the stand-alone generator system is low, the battery-charging device is operated in the MPPT function and provides a continuous charging current. If the generated output power of the electrical power source of the stand-alone generator system is lower than the load power, the electrical power source and the battery supply electric power to the load together. If the battery voltage is lower than a predetermined discharging voltage, the load is cut off. When the generated output power of the electrical power source of the stand-alone generator system is greater than the summation of the load power and the maximum charging power of the constant current charging method for the battery, the battery-charging device is switched and operated in a pulse charging method. In this manner, the battery-charging device is still controlled to execute the MPPT function if the generated output power of the electrical power source of the stand-alone generator system is not greater than the summation of the load power and a maximum charging power of the pulse charging method for the battery. The MPPT function is terminated if the generated output power of the electrical power source of the stand-alone generator system is greater than the summation of the load power and the maximum charging power of the pulse charging method for the battery. The battery-charging device is switched and operated by a constant voltage charging method if the battery voltage is greater than a predetermined constant charging voltage. Accordingly, the utilized generated output power of the electrical power source of the stand-alone generator system of the present invention can be effectively increased.